1. Field of the Invention
The present invention relates to the field of bandgap voltage references.
2. Prior Art
Low noise bandgap references have long been a goal of the industry and have been written about often in the technical journals.
It is well known that a bandgap reference is generated by adding two voltages together, a bipolar transistor Vbe and a delta Vbe. The Vbe has a negative TC and the delta Vbe has a positive TC. When these voltages are added together and their sum is equal to the bandgap voltage, approximately 1.2V, the TC of the sum of the voltages is close to zero.
Since the Vbe is usually close to 600 mV, this means that the delta Vbe must also be in the order of 600 mV. This 600 mV of delta Vbe is hard to generate with a single pair of transistors because it would take very big transistor ratios to do it. Most bandgap references use an amplifier to gain up these transistor ratios. For example, if you have a 10 A to A transistor emitter area ratio (60 mV) you would use an amplifier with a gain of ˜10 to get to 600 mV so you could add this to a 600 mV Vbe to get to the bandgap voltage of 1.2V. This works very well, but the problem with this approach is that the noise is also gained up by 10, which in some cases is undesirable.
U.S. Pat. No. 5,834,926 to Kadanka shows that by using multiple connections of bipolar devices to multiply up the delta Vbe and then gain up the result the noise will be lower. For example, if you can connect two 10 A to A devices and then another 10 A to A device you would have 120 mV of delta Vbe and a gain of only 5 would be necessary to achieve the ˜600 mV. The noise will be lower in this case. This is what is also done in the “stacked bandgap references that use about 1.2V of Vbe and 1.2V of delta Vbe to get an output voltage of ˜2.4V. One of the problems here is that as you stack devices, you may run out of headroom voltage, which is not desirable for low voltage operation.
A particularly well known bandgap reference is commonly referred to as the Brokaw bandgap reference. FIG. 1 presents the circuit diagram of the basic Brokaw bandgap reference. This Figure shows the basic circuit of the reference. In this circuit, resistors R1 and R2 are equal resistors, while the emitter of transistor T1 is much larger than the emitter of transistor T2. The inputs of amplifier A are connected to resistors R1 and R2. The output of amplifier A is the reference voltage Vref, which is also coupled to the bases transistors T1 and T2. Thus the output of the amplifier A seeks a voltage output Vref such that the collector voltages for transistors T1 and T2 and equal, i.e., so that the voltages across and current through the two resistors R1 and R2 are equal. However the transistors T1 and T2 are not of equal size, with transistor T1 being much larger than transistor T2, typically on the order of ten times the size of transistor T2. Thus, while the currents in the two transistors are equal, transistor T1 has a lower base emitter voltage because of its lower current density than transistor T2. Since the bases of transistors T1 and T2 are both coupled to the output voltage Vref, the difference in their base emitter voltages appears across resistor R3. Thus the current through resistor R3 is equal to the difference in base emitter voltages between transistor T2 and transistor T1 divided by the resistance of resistor R3. Also since resistors R1 and R2 are equal, the currents through resistors R1 and R2 and transistors T1 and T2 are made equal by the feedback of the output of amplifier A, the current through resistor R4 is twice the current through resistor R3. From the Ebers-Moll model of a transistor, the difference in base emitter voltages of two transistors (pn junctions) operating at different current densities has a positive temperature coefficient, whereas the base emitter voltage (pn junction) of a single transistor has a negative temperature coefficient. Because the current through resistor R3 has a positive temperature coefficient (PIAT) and the current through transistor T2 is equal to the current through resistor R3, the voltage across resistor R4 also has a positive temperature coefficient (PIAT). Consequently, tracing from the ground connection through resistor R4 and the emitter-base voltage of transistor T2, it may be seen that the output voltage Vref is the sum of the PIAT voltage across resistor R4 and the negative temperature coefficient voltage (CTAT) from the emitter to the base of transistor T2. By appropriate selection of component values, the output voltage Vref can be made equal to the bandgap voltage of the semiconductor material (silicon) with very little temperature sensitivity or power supply sensitivity in the output voltage Vref.
A Brokaw bandgap reference may also be realized by using transistors T1 and T2 of the same emitter area but with unequal resistors R1 and R2. Similarly, circuits are also known which use pn junction diodes as opposed to transistors and/or which use three devices, two to generate the PIAT voltage (the difference in voltage across two pn junctions operating with different current densities) and a third device for providing the negative temperature coefficient of a pn junction.
Numerous variations and improvements have been made in the basic Brokaw bandgap reference. These variations and improvements include techniques for curvature correction to reduce the remaining temperature sensitivity, to broaden the temperature range over which a given temperature sensitivity is achieved, to reduce noise and to achieve similar voltage references using field effect devices. See for instance U.S. Pat. Nos. 5,051,686, 5,619,163, 6,462,526, 6,563,370, 6,765,431 and 7,301,389, all assigned to the assignee of the present invention.
In a Brokaw bandgap reference, the difference in pn junction voltages (base-emitter voltages of transistors T1 and T2 in FIG. 1) operating at different current densities is typically on the order of one-tenth the voltage needed to add to the negative temperature coefficient pn junction voltage to provide the desired temperature insensitive bandgap voltage of approximately 1.23 volts. To be more specific, typically the difference in voltage of two pn junctions operating at different current densities is on the order of 60 millivolts (depending on the current density ratio) while the pn junction voltage is on the order of 600 millivolts. Accordingly, the difference in the pn junction voltages of the two pn junctions operating at different current densities must be voltage amplified by approximately 10 to 1, which in turn amplifies the noise generated by the two transistors. Consequently, while Brokaw type bandgap references still find wide application, there is an increasing need for bandgap references of improved performance, particularly having substantially reduced output noise.